The influence of aging on the mechanisms of liver injury and regeneration was studied in a model of hepatotoxicity induced in 2-, 6-, 12-, 18- and 30-month-old rats by a sublethal dose of thioacetamide (500 mg/kg body weight), a soft nucleophilic and hepatotoxic compound metabolized by the hepatic microsomal FAD monooxygenase system. Samples-blood and hepatocytes-were obtained at 0, 12, 24, 48, 72 and 96 h following thioacetamide intoxication. Parameters of liver injury in serum (NADPH-isocitrate dehydrogenase (ICDH) activity) indicate that the severity of injury was significantly higher in the adult groups (6 and 12 months old) when compared either with the youngest (2 months old) or oldest (18 and 30 months old) groups. Parameters related to biotransformation, such as microsomal FAD monooxygenase, followed mainly the same pattern of age-dependent changes as those observed for injury. The profile of glutathione-S-transferase activity showed an initial induction parallel to liver injury and opposite to the levels of reduced glutathione and protein -SH groups. Enzyme activities and gene expression of the systems involved in the cell endogenous antioxidant defense, such as Mn- and Cu,Zn-superoxide dismutases (SOD), catalase and glutathione peroxidase (GPX) showed significant age-dependent changes that can be summarized as follows: an increase in all enzyme activities and gene expression and a decreased ability to restore the initial activities following 96 h of thioacetamide. We conclude, first, that the gene expression and activity of the enzymes involved in the intracellular antioxidant defense system increased with aging, which can be considered a consequence of the enhanced oxidative state of the cell (decreased in GSH level); and second, that the lower and delayed response in the aged groups significantly influenced the restoration towards normal of GSH and the antioxidant enzyme activities.